Wafer edge expose alignment method

ABSTRACT

Incorporation of a sensor, such as an optical or laser based sensor, into a wafer edge processing unit, such as a WEE unit or mechanism. This sensor enables the WEE unit to be referenced to the wafer edge. Specifically, the sensor can be used to place a WEE unit in a fixed but accurate location at the beginning of the wafer edge expose process. Another approach is to have the WEE drive controller actively follow the edge of the wafer as it rotates during the WEE process, which has the advantage of compensating for any wafer centering errors as well as diameter and placement errors. In yet another approach, the edge sensor is used to sense and track the edge of a previous layer WEE pattern. The sensor can also facilitate the measuring of a distance from a wafer edge to a WEE edge feature.

BACKGROUND

The present invention generally relates to the processing, such as theexposing, of a wafer edge, and more specifically relates to the use of asensor to more precisely process the edge of wafer.

Getting the maximum usable area and highest yield from a wafer iscritical to manufacturing integrated circuits cost effectively. Over theyears, a number of methods have been developed to control the filmstacks that occur on the wafer edge in order to reduce the defects thatthey cause. A common technique is to remove the resist build up, knownas the edge bead, on the edge of the wafer with a combination of solventdispense Edge Bead Remover (EBR) and Wafer Edge Expose (WEE). In recentyears, additional techniques such as wet edge etching and edge scrubprocesses have been deployed as well.

While these processes remove or control films that are the source ofdefects, they do take up some space on the edge of the wafer. Initially,the industry standard for the non-yielding zone at the edge of thewafer, known as the “edge exclusion zone,” was 4 millimeters or more. Asimproved methods for controlling the films in this area becameavailable, it has been possible to reduce this to about 3 millimeters oreven 2 millimeters. The IRTS roadmap calls for reducing the edgeexclusion zone to 1 millimeter by the year 2006.

The edge exclusion radius that must be used for determining waferlayouts and the number of yieldable die sites (also known as the grossdie per wafer) is not actually the physical location of the edge filmremovals. In order for the die to be yielding at the 3 millimeterlocation, the physical edge settings must be closer to the edge thanthat since there are yield loss effects, typically due to CMP processes,that extend some distance from the film removal edge step. In addition,certain films must not be allowed to stack on top of each other or theywill cause particle defects, so there must be room to put differentlayer edges in different locations within the edge zone.

FIG. 1 shows a typical plan for edge exposure settings used for a 3millimeter edge exclusion strategy. In FIG. 1, reference numeral 10identifies all BARC layers, reference numeral 12 identifies thedarkfield WWE layers, reference numeral 14 identifies the brightfieldWWE layers, and reference numeral 16 identifies the contact mask. It canbe seen that the furthest inward physical edge (i.e., the edge 18 of thecontact mask 16) is actually set at 1.9 millimeters.

Control of the edge removal settings with equipment available today istypically ±0.2 millimeter. In order to guarantee no overlaps, eachsetting must be separated by at least 0.4 millimeter with an additional0.4 millimeter allotted for the curvature at the edge of the wafer. Withat least four non-overlapping settings needed (as shown in FIG. 2), thisadds up to a minimum of at least 1.6 millimeters needed for physicaledge settings.

The value of an extra yieldable millimeter at the edge of the wafer isconsiderable. For example, the difference between a 3 millimeter and a 2millimeter edge setting can be worth an additional 10 to 30 die perwafer depending on die size. The increase in gross die per waferrepresents a significant financial value for a typical wafer fabricator.Presently, at a run rate of 2000 wafers a week, the extra 12 die in theexample above are worth an extra $58,000 per week (or $3,000,000 peryear), for no additional processing cost.

In order to meet the IRTS roadmap of 1 millimeter edge exclusion,improvements in two key characteristics must occur:

-   -   1) Improve control of the edge film removal processes to ±0.1        millimeter or less; and    -   2) Reduce the CMP edge yield effect to less than 0.2 millimeter.

The subject of the present invention is to address the need to improvethe edge film removal process to less than ±0.1 millimeter, particularlythe Wafer Edge Expose (WEE) used in photolithography and mask-less waferedge etch processes, such as W and Low K wet etches, using spinprocessors.

The existing method for controlling edge removal settings such as theWEE is to use a mechanical set point on the WEE exposure unit andmeasure the results on the wafer. The edge exposure settings areadjusted on a set frequency and plotted in an SPC control chart.Improved systems utilize a programmable stepper motor to position theWEE unit, and these systems typically have an improved accuracy.

Currently in the industry, the WEE unit position is referenced to theresist coating unit itself and does not account for many key variationssuch as the placement of the wafer on the unit, variations in waferdiameter, and placement of previous WEE layers. FIG. 2 shows a top viewof a wafer 20 and a Wafer Edge Expose unit 22, wherein the wafer isrotated (as represented by arrow 24) under the Wafer Edge Expose unit22.

FIG. 3 provides a cross sectional view of the wafer 20 and Wafer EdgeExpose unit 22. Specifically, reference numeral 26 identifies the vacuumchuck which holds the wafer 20, reference numeral 28 identifies theresist which is on the wafer 20, reference numeral 30 identifies theresist edge bead, reference numeral 32 identifies UV light which isproduced by the edge expose unit 22, and reference numeral 34 identifiesthe wafer edge expose drive motor and controller. FIG. 3 alsoillustrates the many different tolerances or variations which can add upto an unacceptable WEE placement error. Specifically, reference numeral36 identifies the wafer centering tolerance, reference numeral 38identifies the wafer edge expose unit tolerance (i.e., with regard toplacement relative to a centerline 39 of the chuck), and referencenumeral 40 identifies the wafer diameter tolerance.

OBJECTS AND SUMMARY

An object of an embodiment of the present invention is to provide amethod and apparatus for better aligning a wafer edge processing unit,such as a wafer edge expose unit, with regard to a wafer edge.

Another object of an embodiment of the present invention is to provide amethod and apparatus which compensates for tolerances when aligning awafer edge processing unit, such as a wafer edge expose unit, withregard to a wafer edge.

Yet another object of an embodiment of the present invention is toprovide a method and apparatus for dynamically tracking a wafer edge,such as during wafer edge processing.

Briefly, and in accordance with at least one of the foregoing objects,an embodiment of the present invention provides for the incorporation ofa sensor, such as an optical or laser based sensor, into a wafer edgeprocessing unit, such as a WEE unit or mechanism. This sensor enablesthe wafer edge processing unit to be referenced to the wafer edge.Specifically, the sensor can be used to place a WEE unit in a fixed butaccurate location at the beginning of the WEE process. Another approachis to have the WEE drive controller actively follow the edge of thewafer as it rotates during the WEE process, which has the advantage ofcompensating for any wafer centering errors as well as diameter andplacement errors. In another approach, the edge sensor is used to senseand track the edge of a previous layer WEE pattern. The sensor can alsofacilitate the measuring of a distance from a wafer edge to a WEE edgefeature.

BRIEF DESCRIPTION OF THE DRAWINGS

The organization and manner of the structure and operation of theinvention, together with further objects and advantages thereof, maybest be understood by reference to the following description, taken inconnection with the accompanying drawings, wherein:

FIG. 1 shows a typical plan for edge exposure settings used for a 3millimeter edge exclusion strategy;

FIG. 2 provides a top view of a Wafer Edge Expose Process;

FIG. 3 provides a side view of that which is shown in FIG. 2;

FIG. 4 is similar to FIG. 3, but shows an embodiment of the presentinvention, wherein a laser and sensor are incorporated into the waferedge expose unit in a feedback loop so the wafer edge expose unitfollows the wafer edge;

FIG. 5 is similar to FIG. 4, but shows another embodiment of the presentinvention, wherein the laser and sensor are incorporated into the waferedge expose unit in a feedback loop so the wafer edge expose unitfollows a previous WEE layer edge; and

FIG. 6 provides a flow chart of a method of measuring the distance froman edge of a wafer to a WEE edge feature, where the method is inaccordance with an embodiment of the present invention.

DESCRIPTION

While the invention may be susceptible to embodiment in different forms,there are shown in the drawings, and herein will be described in detail,specific embodiments of the invention. The present disclosure is to beconsidered an example of the principles of the invention, and is notintended to limit the invention to that which is illustrated anddescribed herein.

The present invention provides a method and apparatus for betteraligning a wafer edge processing unit, such as a wafer edge expose unit,with regard to a wafer edge. As discussed above, FIGS. 2 and 3illustrate a wafer edge expose unit 22 and a wafer 20, duringprocessing, as currently used in the industry. The WEE unit position isreferenced to the resist coating unit itself and does not account formany key variations such as the placement of the wafer on the unit,variations in wafer diameter, and placement of previous WEE layers.

The present invention provides for the incorporation of a sensor into awafer edge processing unit, such as a wafer edge expose unit, forsensing an edge of the wafer. A sensor method that does not physicallycontact the wafer, such as commonly available light or capacitance basedsensors, is preferred. The present invention may also provide for theuse of a light or energy-emitting source, such a laser, along with useof the sensor, wherein the sensor senses the light or energy emittedfrom the source. Regardless, by sensing the edge of the wafer, thepresent invention compensates for tolerances when aligning the waferedge expose unit relative to the wafer edge.

FIG. 4 is similar to FIG. 3, but illustrates a specific embodiment ofthe present invention, wherein a sensor 50 is incorporated into thewafer edge expose unit 22. More specifically, reference numeral 50 inFIG. 4 identifies a sensor as well as possibly a corresponding lightsource. For example, one embodiment provides that 50 is a combined laserand reflectance sensor. Another embodiment provides that 50 is anambient light CCD sensor or a capacitance sensor. Still anotherembodiment provides that 52 is a light source positioned on one side ofthe wafer and a corresponding sensor 50 is positioned on the oppositeside of the wafer, where the sensor 50 senses the light from the lightsource 52. Regardless, preferably the wafer edge expose unit 22 ispositioned depending on what is sensed by the sensor 50 with regard tothe wafer edge. As such, the wafer edge expose unit 22 is referenced tothe wafer edge, and tolerances are compensated for when aligning thewafer edge expose unit 22.

There are several modes that the sensor could be used in. One approachwould be to place the WEE unit 22 in a fixed but accurate location atthe beginning of the WEE process (reference numeral 60 in FIG. 4identifies the wafer edge to wafer edge expose unit offset). Anotherapproach would be to have the WEE drive controller 34 a actively followthe edge of the wafer 20 as it rotates during the WEE process, which hasthe advantage of compensating for any wafer centering errors (i.e.,dimension 36 in FIG. 3) as well as diameter and placement errors (suchas dimension 40 in FIG. 3). In other words, the wafer edge isdynamically tracked and followed during processing.

In another approach, the edge sensor 50 is used to sense and track theedge of a previous layer WEE pattern as shown in FIG. 5. In FIG. 5, line62 identifies a previous layer WEE edge, and reference numeral xxidentifies the previous WEE edge to next WEE edge offset.

Another use of a sensor equipped WEE unit with an accurate drive unitlike a stepper motor is to measure the distance from the edge of thewafer to a WEE edge feature. To do this, as shown in FIG. 6, the sensoris driven from the feature to the edge while counting the motor steps.The sensor signal is measured to detect the WEE layer edges or waferedge. The stepper motor counts are then multiplied by the distance perstep to calculate the total distance between edge features.

The present invention is generally directed at compensating for waferand machine tolerances to accurately place the wafer edge expose unitrelative to the wafer edge (or a previous WEE layer) so that the IRTSroadmap of 1 millimeter edge exclusion can be achieved. However, thepresent invention can also be employed in connection with other non-WEEedge processing, such as wet edge etch on a spin processor or a directedplasma edge etch.

While embodiments of the present invention are shown and described, itis envisioned that those skilled in the art may devise variousmodifications of the present invention without departing from the spiritand scope of the appended claims.

1. A wafer edge expose apparatus for exposing an edge of a wafer, saidapparatus comprising: means for holding and rotating the wafer; a waferedge expose unit which includes a sensor which is configured to senseand teh wafer edge expose unit is configured to expose the edge of thewafer; and means for moving the wafer edge expose unit depending on whatis sensed by the sensor.
 2. A wafer edge expose apparatus as recited inclaim 1, wherein the means for holding and rotating the wafer comprisesa wafer chuck.
 3. (canceled)
 4. A wafer edge expose apparatus as recitedin claim 1, wherein the sensor of the wafer edge expose unit comprisesat least one of an optical sensor, a capacitance sensor and areflectance sensor.
 5. A wafer edge expose apparatus as recited in claim1, further comprising a laser and wherein said sensor of the wafer edgeexpose unit comprises a reflectance sensor configured to sense saidlaser.
 6. A wafer edge expose apparatus as recited in claim 1, furthercomprising a light source disposed proximate one side of the wafer andwherein the sensor is disposed proximate another side of the wafer.
 7. Awafer edge expose apparatus as recited in claim 1, wherein the means formoving the wafer edge expose unit comprises a drive motor and acontroller connected to said drive motor, wherein said sensor of thewafer edge expose unit is connected to said controller.
 8. A wafer edgeexpose apparatus as recited in claim 7, wherein the controller isconfigured to position said wafer edge expose unit depending on what issensed by the sensor.
 9. A wafer edge expose apparatus as recited inclaim 7, wherein the controller is configured to move said wafer edgeexpose unit depending on what is sensed by the sensor during processing,while the wafer is rotating and said wafer processing unit is active.10. A method of exposing the edge of a wafer, said method comprising:providing a wafer edge expose unit which includes a sensor; using thesensor of the wafer edge expose unit to sense the edge of the wafer;using the wafer edge expose unit which includes the sensor to expose theedge the wafer and moving the wafer edge expose unit depending on whatis sensed by the sensor.
 11. A method as recited in claim 10, furthercomprising using a wafer chuck to hold and rotate the wafer. 12.(canceled)
 13. A method as recited in claim 10, wherein the step ofproviding a sensor comprises providing at least one of an opticalsensor, a capacitance sensor and a reflectance sensor on the wafer edgeexpose unit.
 14. A method as recited in claim 10, further comprisingproviding a laser, wherein the step of providing a sensor comprisesproviding a reflectance sensor, pointing the laser toward the wafer andusing the sensor to sense the laser.
 15. A method as recited in claim10, further comprising providing a light source disposed proximate oneside of the wafer and positioning said sensor proximate another side ofthe wafer.
 16. A method as recited in claim 10, further comprisingpositioning said wafer edge expose unit depending on what is sensed bythe sensor.
 17. A method as recited in claim 10, further comprisingmoving said wafer edge expose unit depending on what is sensed by thesensor during processing, while the wafer is rotating and said waferexpose unit is active.